Driving while intoxicated on cannabis is a major and growing public health problem. Driving safety is an important issue in the United States as more than 3 million persons are reported injured and >40,000 die annually in motor vehicle crashes. Traffic accidents are the greatest single cause of death in 5-32 year-olds; many are due to intoxicated drivers. Cannabis (CNB) is a commonly abused drug whose use cuts across social class, is linked to cognitive impairment, is a major contributor to intoxication-related accidents either alone or with alcohol. However, CNB intoxication is little studied in relation to driving compared to alcohol. Not only does the current NHTSA Strategic Plan for Behavioral Research prioritize understanding how drugs other than alcohol contribute to traffic crashes, it has recentl become more pressing to understand the effects of CNB because of increasing rates of legalized medical and/or recreational use, that will likely result in more CNB intoxicated drivers. Social and legal policy will be unable to effectively address the many concerns about driving safety raised by more frequent and widespread use of CNB without new research to better determine the parameters within which CNB use does, or does not, increase automobile accident risk. Our purpose is to better describe specific, driving-related cognitive impairments caused by acute CNB intoxication, their persistence over time, underlying functional brain anatomy, and relationship to performance on a state-of the art validated simulated driving task in which we have prior experience. In a randomized, counterbalanced, double-blinded fashion, we will administer two CNB doses and placebo of smoked cannabis (paced inhalation using a vaporizer) to 48 regular and 48 occasional cannabis users on 3 separate occasions. Following CNB dosing we will assess cognitive and driving impairment longitudinally for several hours using a combination of fMRI and neuropsychological tests, to clarify relationships between subjective and objective measures of intoxication and of impairment, that include expert assessment of THC and its metabolite levels in blood and saliva.